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CO oxidation mechanism on a MgO(1 0 0) supported PtxAu3−x clusters
Applied Surface Science 356 (2015) 282–288
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CO oxidation mechanism on a MgO(1 0 0) supported Ptx Au3−x clusters Wei Zhang a,b , Rong Cui c , Hao Wu c , Jiqin Zhu b,∗∗ , Daojian Cheng a,c,∗ a
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China c Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 25 June 2015 Received in revised form 27 July 2015 Accepted 10 August 2015 Available online 12 August 2015 Keywords: CO oxidation Reaction mechanism Ptx Au3−x clusters DFT calculation
a b s t r a c t In this work, we systematically studied the CO oxidation on a MgO(1 0 0) supported Ptx Au3−x clusters via Langmuir–Hinshelwood (LH), trimolecular Langmuir–Hinshelwood (3LH), and Eley–Rideal (ER) mechanisms on the basis of density functional theory (DFT) calculations. It is found that the Pt2 Au1 /MgO cluster possesses the highest catalytic activity relevant to CO oxidation, which is considered to be the best catalyst among these supported subnanometer Ptx Au3−x clusters. We expect that computational screening of mechanism-dependent properties of sub-nanocatalysts in this work could be useful in the design of highly efficient nanocatalysts. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nanosized gold clusters have attracted great interest over the past few decades owing to their unusual catalytic properties not seen in bulk gold. For example, nanosized gold clusters exhibit high catalytic activities toward the carbon monoxide oxidation [1,2], selective oxidation of olefin and alcohol [3], synthesis of hydrogen peroxide [4], and water–gas shift reaction [5]. Among various reactions catalyzed by gold clusters, CO oxidation has attracted great attention and become a prototypical reaction for examining the activities of nanosized gold clusters [6–10]. More interestingly, the activity of gold clusters relevant to CO oxidation is strongly dependent on the cluster size. After Haruta’s discovery that supported gold nanoclusters are active in CO oxidation at low temperature [2], a lot of studies have flourished on this topic. More interestingly, the active catalyst for CO oxidation can be subnanometer species containing around 10 Au atoms [11]. Landman et al. found that the supported Au8 cluster is highly active for CO oxidation by both experimental and theoretical studies [12,13]. In addition, Skorodumova et al. studied the catalytic activity of MgO(1 0 0)-supported Au1–4 clusters toward CO oxidation by density functional theory
∗ Corresponding author at: State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Zhu), [email protected] (D. Cheng). http://dx.doi.org/10.1016/j.apsusc.2015.08.081 0169-4332/© 2015 Elsevier B.V. All rights reserved.
(DFT) calculations [14]. These findings can trigger our studies on CO oxidation on supported small (subnanometer) Au clusters. In recent years, bimetallic clusters have attracted a lot of attention, since the introduction of a second metal into the single metal cluster can improve the catalytic activity of the clusters by the synergetic effect of two metals [15–17]. Notably, Au-based bimetallic clusters by the mixing of Au with other metals, such as Pt [15,18], Pd [16], Ag [17], Ir [19], and Cu [20], have been used to improve the catalytic efficiency, stability, and selectivity of pure Au clusters for CO oxidation. Among these systems, Pt–Au bimetallic clusters have been found experimentally to show excellent catalytic performance for CO oxidation [21–24]. However, only a few studies have been focused on the catalytic properties of bimetallic subnanometer clusters for CO oxidation [16,17,25–27]. The pioneering work of Fortunelli et al. studied the catalytic properties of Agx Au3−x clusters supported on the MgO(1 0 0) surface [17], and demonstrated that Ag2 Au1 cluster is a good catalyst for CO oxidation in terms of efficiency and stability. In addition, Ding et al. studied the catalytic properties of CO oxidation on free Ptm Aun (m + n = 4) clusters by DFT calculations [28], and found that free Pt–Au clusters are more active than the corresponding pure Pt4 cluster for CO oxidation. To the best of our knowledge, the theoretical understanding of the reaction mechanism of CO oxidation on supported subnanometer Pt–Au clusters is still lacking and the details of how the support and cluster composition affect the reaction mechanisms is also an open question. DFT calculation is an established tool to evaluate the catalytic properties of the clusters via different reaction mechanisms for CO oxidation [29]. In general, two well-established reaction
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288
mechanisms, namely Langmuir–Hinshelwood (LH) and Eley–Rideal (ER), are considered for CO oxidation [17,30–32]. For the LH mechanism, the coadsorbed CO and O2 firstly form a peroxo-type complex intermediate, i.e., CO + O2 → OOC → O + CO2 , then the intermediate OOCO is critical for CO oxidation, while for the ER mechanism, CO reacts directly with activated O2 , but the precondition is that O2 is easily dissociated to O atom, i.e., CO + O → CO2 . Moreover, Zeng et al. found that the coadsorbed CO molecule at a unique triangular Au3 active site can act as a promoter for the scission of an O O bond, leading to the spontaneous formation (due to the extremely low energy barrier) of two CO2 molecules as product, i.e., 2CO + O2 → OCOOCO → 2CO2 , which is named as the trimolecular Langmuir–Hinshelwood (3LH) mechanism [33]. On the theoretical side, all the three mechanisms could take place on the catalyst. However, detailed accurate studies involving all the three mechanisms for CO oxidation reaction on a selected system have been largely overlooked. Thus, there is a need to explore all the possible intermediate products and reaction pathways for CO oxidation reaction on heterogeneous subnanometer catalysts, such as supported subnanometer Pt–Au clusters. In this work, a complete and systematic study of CO oxidation on the Ptx Au3−x trimers supported on the MgO(1 0 0) surface via LH, 3LH, and ER mechanisms are studied by DFT calculations. In particular, the adsorption energies of reactants (O2 , CO, 2CO, CO + O2 , and 2CO + O2 ), intermediates (OOCO, CO + OOCO, and COOOCO), and energy barriers of three mechanisms on the MgO(1 0 0)-supported Ptx Au3−x trimmers are calculated. The effect of support and cluster composition on the reaction mechanisms is discussed.
2. Computational details DFT calculations were performed using the PWSCF (PlaneWave Self-Consistent Field) plane wave code in the Quantum ESPRESSO package [34]. All the calculations were carried out by the spin-polarized generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) [35] XC-functionals together with ultrasoft pseudopotentials (see Supporting Information for details) [36]. The ultrasoft pseudopotentials used in this work were generated with a Scalar-relativistic calculation, which include the nonlinear core correction (NLCC) and relativistic effect. The MgO(1 0 0) surface was modeled in a supercell approach with 2ML thick 3 × 3 MgO slab. In order to test the convergence of the adsorption energies of Ptx Au3−x clusters on MgO slab with different layers, the adsorption energies of Ptx Au3−x cluster on MgO with different layers were calculated, as listed in Table S1. It is found that the energies changes are less than 0.03 eV using two or three layers for MgO slab, which is within the DFT error bar. Therefore, the 2ML thickness for MgO slab is acceptable and used in this work. The repeated slabs were separated by 20 A˚ vacuum region to avoid the interaction between two successive slabs. All the MgO atoms in the unit cell were kept frozen during structural optimizations, since the flat MgO substrate is quite rigid [17,37–39]. Spin-polarized calculations were performed here by using values of 40 and 400 Ry as the energy cut-off for the selection of the plane waves for the description of the wave function and the electronic density, respectively, which are considered to be reasonable after the convergence test (see Fig. S1). The Brillouin zone sampled at the Gamma-point only, and the geometry of the cluster upon or without adsorption is optimized until the total energy is converged to 10−6 eV. We calculated the adsorption energies of CO and O2 according to the equation: Eads = Etotal − Ecluster/MgO − Eadsorbate
(1)
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Furthermore, the coadsorption energies of CO and the molecular or atomic oxygen were calculated as: Ecoads = Etotal − Ecluster/MgO − Eco − Eo2 (or O)
(2)
In the above equations, Etotal , Ecluster/MgO , Eabsorbate , Eco , and EO2 (or O) correspond to the electronic energies of adsorbed species on the supported cluster, the bare supported cluster, a gas-phase adsorbate, gaseous CO and gaseous O2 (or O), respectively. The climbing-image nudged elastic band (CI-NEB) method [40] was adopted here to determine the reaction pathway for the CO oxidation. In the CI-NEB calculations, the reaction paths are interpolated by fitting a cubic polynomial through all images on each path guided by the force tangent to the reaction coordinate at each image. Once a minimum-energy path is determined, the transition state is located [41]. The energies along the paths were referred to the sum of energies of the noninteracting cluster and reactant(s), and the barrier heights along them representing the activation energies were calculated as Eb = E TS − E IS , ETS
(3) EIS
where and are the total energies of the transition and initial states, respectively. 3. Results and discussion 3.1. The structure of the cluster In this work, we optimized the triangular clusters of the supported Ptx Au3−x clusters with two configurations: (a) the planar configuration, (b) the “metal-on-top” configuration, as shown in Fig. S2. Our results certify that the planar configuration on the MgO surface is unstable, which would be changed into the “metal-ontop” one after optimization. This finding is in excellent agreement with many previous works [18,25,42,43]. In order to find the stable structure for these supported clusters, all the “metal-on-top” configurations of the Ptx Au3−x trimmers on the MgO surface were studied. The available adsorption configurations and the corresponding adsorption energies of the Ptx Au3−x trimers on the MgO surface are shown in Fig. S3. Only one “metal-on-top” configuration is found for the supported Au3 or Pt3 clusters, while two different “metal-on-top” configurations are found for the supported Au2 Pt1 and Au1 Pt2 clusters. In addition, all the most stable adsorption configurations and the corresponding adsorption energies of the Ptx Au3−x trimmers on the MgO surface are shown in Fig. 1. The adsorption energies for the Pt3 , Pt2 Au1 , Au2 Pt1 , and Au3 clusters on the MgO(1 0 0) surface are −2.73, −2.98, −2.18, and −1.42 eV, respectively, indicating that the adsorption strength of these clusters on the MgO(1 0 0) surface follows the order: Pt2 Au1 > Pt3 > Au2 Pt1 > Au3 . It means that alloying with Pt can increase the adsorption strength of the Au3 cluster on the MgO(1 0 0) surface. Another crucial feature of the catalyst is its stability under reaction conditions. To check this aspect of the supported Ptx Au3−x trimers, the energy barriers for breaking Ptx Au3−x CO and Ptx Au3−x O2 complexes on the MgO surfaces were calculated, as listed in Table 1. The corresponding initial and final configurations are shown in Fig. S4. The energy barriers for breaking the bare Pt3 , Pt2 Au1 , Au2 Pt1 , and Au3 clusters were also calculated for comparison, as listed in Table 1. It is found that the difficulty for breaking the supported Ptx Au3−x CO and Ptx Au3−x O2 complexes follows the order: Pt3 > Pt2 Au1 > Au2 Pt1 > Au3 . Upon the adsorption of CO/O2 , the supported Pt3 and Pt2 Au1 clusters are found to be still reasonably stable, whereas the supported Au3 and Pt1 Au2 clusters could break easily. Our results show that Pt-rich trimers should have a longer lifetime under reaction conditions of CO oxidation.
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Fig. 1. The most favorable adsorption configurations of (a) Au3 /MgO, (b) Pt1 Au2 /MgO, (c) Pt2 Au1 /MgO, and (d) Pt3 /MgO clusters. Ead denotes their adsorption energy on Mg(1 0 0) and M is their preferred spin multiplicity; Au, Pt, Mg, and O atoms are represented by yellow, blue, green, and red spheres, respectively (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Table 1 The energy barriers (Eb in eV) for breaking the Ptx Au3−x /MgO clusters into smaller pieces with or without the adsorption of CO or O2. Eb
Ptx Au3−x
Ptx Au3−x CO
Ptx Au3−x O2
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
0.79 1.96 2.45 2.88
0.53 0 2.24 9.53
1.14 1.43 1.96 2.46
3.2. The adsorption of CO and O2 As illustrated in Fig. 2a, the surface of a supported Ptx Au3−x trimer is a triangle. On the triangle, seven different adsorption sites are possible: three T (on the top of three vertex atoms) sites, three B (on the bridge between two vertex atoms) sites, and one H (on the hollow among three vertex atoms) site. We start by conducting a preliminary investigation on CO adsorption on the supported Ptx Au3−x trimers at each different composition. For CO adsorption, the molecule prefers the “end-on” configurations with C atom bind to the Pt or Au atoms, as shown in Fig. 2b. Except the Au2 Pt1 /MgO cluster, which has three different T sites, the other supported Ptx Au3−x clusters have only two different T sites. All the adsorption sites and the corresponding adsorption energies of CO and 2CO on the supported Ptx Au3−x trimmers are listed in Table S2. All the most stable adsorption configurations and the corresponding adsorption energies of CO on the supported Ptx Au3−x trimmers are listed in Table 2. It is found that the T1 site is the most stable one for CO adsorption on the Au3 /MgO cluster with the adsorption energy of −1.39 eV. Accordingly, the T3 site is the most favorable one for CO adsorption on the Au2 Pt1 /MgO cluster with the adsorption energy of −1.96 eV. In addition, the T2 site is the most stable one for CO adsorption on the Pt2 Au1 /MgO and Pt3 /MgO clusters with the adsorption energies of −2.25 and −3.57 eV, respectively. For O2 adsorption on these supported clusters, the O2 molecule prefers the “side-on” configurations on B sites and the “end-on” configurations on the T and H sites, as shown in Fig. 2b. The available adsorption configurations and the corresponding adsorption energies of O2 on the supported Ptx Au3−x trimmers are summarized in Table S3. It is found that the B1 configuration is the most stable one for O2 adsorption on the Au3 /MgO and Au2 Pt1 /MgO clusters with the adsorption energies of −1.03 and −1.05 eV, respectively. In contrast, the T2 configuration is the most stable one for O2 adsorption on the Pt2 Au1 /MgO cluster with the adsorption energy of −1.19 eV. For O2 adsorption on the Pt3 /MgO cluster, the B2 configuration is the most stable one with the adsorption energy of −2.16 eV. All the most stable adsorption configurations and the corresponding adsorption energies of O2 on the supported Ptx Au3−x trimmers are listed in Table 2. In order to identify the spin states of the O2 adsorption intermediates involved in the reaction pathway, the spin states of O2 in the ground state and the reaction
pathway for the Pt2 Au1 /MgO cluster are listed in Table S4. It is found that the interaction of O2 with the metal atoms can shift the triplet ground state 3 O2 into the doublet 2 O2 − (or singlet 1 O2 − ), which is well-known to be highly reactive [44]. 3.3. CO oxidation For CO oxidation, a bimolecular Langmuir–Hinshelwood (LH) mechanism is investigated firstly. The potential energy profile and configurations for CO oxidation on Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters with the LH mechanism are shown in Fig. 3a–d, respectively. For clarity, the corresponding adsorption energies and energy barriers on these clusters with the LH mechanism are listed in Table 3. As mentioned above, both CO and O2 molecules could adsorb on the Ptx Au3−x /MgO clusters, which would provide the initial stage for the catalytic reaction of CO oxidation. After the check, the favorable coadsorption site for CO and O2 molecules on the Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters is found to be T1 for O2 and T3 for CO, as listed in Table 2. Moreover, the favorable coadsorption site for CO and O2 molecules on the Au3 /MgO cluster are found to be T1 for O2 and T1 for CO (see Table 2). In the following, CO and O2 move closer to form the intermediate state OCOO* with a C O single bond and a peroxide O O bond, followed by crossing of the first energy barrier (TS1). The corresponding energy barriers (TS1) are found to be 0.17, 0.63, 0.47, and 0.50 eV for the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters, respectively. It is noted that the energy barrier for the formation of the intermediate state OCOO* (TS1) on the Au3 /MgO cluster is lowest among these clusters. In the third step, the peroxide O O bond dissociates into two O atoms by crossing the second energy barrier (TS2). For the Au3 /MgO cluster, the energy barrier (TS2) is 0.29 eV, which is bigger than its energy barrier for the formation of OCOO* (TS1). For the Au2 Pt1 /MgO cluster, the value of energy barrier (TS2) is 0.11 eV, which is much lower than that of its first energy barrier (TS1). In contrast, the calculated energy barriers (TS2) are zero for the Pt2 Au1 /MgO and Pt3 /MgO clusters. It means that CO2 molecule is easily desorbed from the supported Pt-rich clusters without energy barrier. For CO oxidation on these supported clusters with the LH mechanism, the Au3 /MgO cluster has the lowest energy barrier for CO oxidation on these supported subnanometer Ptx Au3−x clusters, and the activity of CO oxidation follows the order: Au3 /MgO > Pt2 Au1 /MgO > Pt3 /MgO > Pt1 Au2 /MgO. The trimolecular Langmuir–Hinshelwood (3LH) mechanism was also investigated, which is also called CO self-promoting oxidation mechanism [33]. In this mechanism, the coadsorbed CO molecule at a unique triangular Au3 active site can act as a promoter for the scission of an O O bond, leading to the spontaneous formation of two CO2 molecules. In order to investigate the 3LH reaction mechanism, the adsorption of two CO molecules on these supported clusters was also calculated, as listed in Table 2. For the
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Fig. 2. (a) Atomic adsorption sites for the triangular facet of Pt1 Au2 cluster. For the nomenclature of adsorption sites of T1, T2, T3, B1, B2, B3, H, please see the text. (b) Adsorption mode of CO on the T site with “end-on” configuration, adsorption mode of O2 on the B1 site with “side-on” configuration and the H site with “end-on” configuration. Table 2 The most stable adsorption sites and the corresponding adsorption energies (Ead in eV) of CO and O2 , and coadsorption of 2CO, CO + O2 , and 2CO + O2 on these Ptx Au3−x /MgO clusters. Sites labeled “N.A.” are unstable.
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
Adsorption site Ead Adsorption site Ead Adsorption site Ead Adsorption site Ead
O2
CO
2CO
CO + O2
2CO + O2
B1 −1.03 B1 −0.88 T2 −1.19 B1 −2.16
T1 −1.39 T3 −1.96 T2 −2.25 T2 −3.57
T2 + T3 −2.09 T1 + T3 −3.58 T2 + T3 −4.19 T1 + T2 −6.18
T1 + T1 −1.60 T1 + T3 −2.65 T1 + T3 −2.57 T1 + T3 −4.13
N.A. – N.A. – T1 + T2 + T3 −6.45 B1 + T1 + T3 −6.42
Fig. 3. Potential energy profile and configurations for CO oxidation with the Langmuir–Hinshelwood (LH) mechanism on the (a) Au3 /MgO, (b) Pt1 Au2 /MgO, (c) Pt2 Au1 /MgO, and (d) Pt3 /MgO clusters. Table 3 Calculated relative adsorption energies (in eV) of initial adsorption (CO), coadsorption (CO + O2 ), intermediate (OCOO), the final (CO2 + O) states, and the energy barriers (in eV) of the first (TS1) and second (TS2) transition states of CO oxidation reaction with the Langmuir–Hinshelwood (LH) mechanism on these Ptx Au3−x /MgO clusters.
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
CO
CO + O2
OOCO
O + CO2
TS1(->OCOO)
TS2(->CO2 + O)
−1.39 −2.40 −2.25 −3.57
−1.60 −2.65 −2.57 −4.13
−1.59 −2.11 −1.79 −3.54
−3.55 −2.98 −3.21 −5.06
0.17 0.63 0.47 0.50
0.29 0.11 0 0
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Fig. 4. Potential energy profile and configurations for CO oxidation with the trimolecular Langmuir–Hinshelwood (3LH) mechanism on the (a) Pt1 Au2 /MgO and (b) Au3 /MgO clusters. Table 4 Calculated relative adsorption energies (in eV) of initial adsorption (CO), the coadsorption (2CO/2CO + O2 ), intermediate (CO + OCOO/COOOCO), the final (2CO2 ) states, and the energy barriers (in eV) of the first (TS1) and second (TS2) transition states of CO oxidation reaction with the trimolecular Langmuir–Hinshelwood (3LH) mechanism on the Pt2 Au1 /MgO and Pt3 /MgO clusters.
Pt2 Au1 /MgO Pt3 /MgO
CO
2CO
2CO + O2
CO + OOCO
COOOCO
2CO2
TS1(->OCOO)
TS2(->COOOCO)
−2.25 −3.57
−4.19 −6.18
−6.45 −6.42
−5.74 −5.74
−5.14 −5.31
−6.03 −8.66
0.69 0.71
0.67 1.29
Fig. 5. Potential energy profile and configurations for CO oxidation with the Eley–Rideal (ER) mechanism on the (a) Au3 /MgO, (b) Pt1 Au2 /MgO, (c) Pt2 Au1 /MgO, and (d) Pt3 /MgO clusters.
Au3 /MgO and Pt2 Au1 /MgO clusters, the T2 and T3 sites are the stable adsorption ones for the coadsorption of two CO molecules, while the most stable adsorption sites are T1 and T3 ones for the Au2 Pt1 /MgO and Pt3 /MgO clusters. In addition, we also calculated the coadsorption of two CO molecules and one O2 molecule. It is found that the coadsorption of two CO molecules can promote the
adsorption of O2 for the Pt2 Au1 /MgO cluster, but the coadsorption of two CO molecules can inhibit the adsorption of O2 molecule for the Pt3 /MgO cluster, as listed in Table 2. It means that the coadsorption of two CO molecules on the surface of small clusters can significantly change the structure and the reactivity of the cluster toward the O2 molecule. Moreover, the coadsorption of two CO
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288 Table 5 Calculated relative adsorption energies (in eV) of initial adsorption (O2 ), the coadsorption (2O), the final (2CO2 ) states, and the energy barriers (in eV) of the first (TS1) transition states of CO oxidation reaction with the Eley–Rideal (ER) mechanism on the Ptx Au3−x /MgO clusters.
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
O2
O O
2CO2
TS1(->2O)
−1.03 −0.88 −0.38 −2.16
0.16 −0.11 −3.11 −3.11
−5.99 −6.65 −6.03 −8.66
2.12 0.96 0.22 0.34
molecules and one O2 molecule on the Au3 /MgO and Au2 Pt1 /MgO clusters is impossible, meaning that the 3LH reaction mechanism is not available for the Au3 /MgO and Au2 Pt1 /MgO clusters. In contrast, the 3LH mechanism can occur on Au clusters with tens of atoms [33], indicating that the size of the cluster plays an important role in this mechanism. The potential energy profile and configurations for CO oxidation on the Pt2 Au1 /MgO and Pt3 /MgO clusters with the 3LH mechanism are shown in Fig. 4a and b, respectively. For clarity, the corresponding adsorption energies and energy barriers on these supported clusters with the 3LH mechanism are listed in Table 4. It is found that the coadsorbed CO molecule cannot promote the O O bond breaking via 3LH mechanism for the Pt2 Au1 /MgO and Pt3 /MgO clusters, since the activation energy of O O bond dissociation via 3LH mechanisms is 0 eV. The energy barriers for the 3LH mechanism are bigger than that of the previous LH mechanism. Our results show that the 3LH mechanism is not a universally applicable mechanism for these supported subnanometer Ptx Au3−x clusters. We also investigated the Eley–Rideal (ER) mechanism on these supported subnanometer Ptx Au3−x clusters. For the ER mechanism, the following step after O2 breaking is the reaction of an O atom with CO, in which the dissociation of O2 on the clusters is the critical process. The potential energy profile and configurations for CO oxidation on the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters with the ER mechanism are shown in Fig. 5a–d, respectively. The corresponding adsorption energies and energy barriers on these supported clusters with the ER mechanism are also given in Table 5. It is found that the dissociation energy barriers of O2 (TS1) are 2.12, 0.96, 0.22, and 0.34 eV for the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters, respectively. It means that the dissociation of O2 is really difficult on the supported Au-rich clusters, which is in excellent agreement with the previous work [45]. In the following, the O atom reacts with the gas CO molecule by crossing the second energy barrier (TS2). The calculated TS2 energy barriers are zero for these supported subnanometer Ptx Au3−x clusters. It means that CO2 molecule is easily formed without energy barrier in the ER mechanism. For CO oxidation on these supported clusters with the ER mechanism, the Pt2 Au1 /MgO cluster has the lowest energy barrier for CO oxidation on these supported subnanometer Ptx Au3−x clusters, and the activity of CO oxidation follows the order: Pt2 Au1 /MgO > Pt3 /MgO > Au2 Pt1 /MgO > Au3 /MgO. To conclude this section, we briefly discuss the three different reaction mechanisms, namely LH, 3LH, and ER, for CO oxidation on the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters. It is found that 3LH mechanism cannot promote the CO oxidation reactivity on the supported subnanometer Pt2 Au1 and Pt3 clusters. For the LH mechanism, the Au3 /MgO cluster has the lowest energy barrier of 0.29 eV for CO oxidation on these supported subnanometer Ptx Au3−x clusters. For the ER mechanism, the Pt2 Au1 /MgO cluster has the lowest energy barrier of 0.22 eV for CO oxidation on these supported subnanometer Ptx Au3−x clusters. Considering the LH, 3LH, and ER mechanisms, the Pt2 Au1 cluster possesses the highest catalytic activity relevant to CO oxidation, which is considered to be the best catalyst among these supported subnanometer Ptx Au3−x clusters.
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It is well-known that the activity of the catalyst to a specific reaction is largely dictated by the local electronic environments of the catalyst. In this work, the Pt2 Au1 /MgO cluster possesses the higher catalytic activity relevant to CO oxidation, compared with the Au3 /MgO cluster. To understand the reason, the Löwdin electronic population was used to analyze the local electronic environments of the Ptx Au3−x /MgO clusters. Fig. S5 shows the number of the total charge of Pt and Au atoms for these supported Ptx Au3−x clusters. It is found that more charge transfer from Au to Pt atoms takes place in Pt–Au bimetallic clusters, compared with the pure Au3 and Pt3 clusters. It is well-known that the charge transfer between Au and Pt atoms can improve the activity of the Pt–Au bimetallic clusters, which may be the reason that the supported Pt2 Au1 cluster is more active than that of the supported Pt3 cluster. 4. Conclusion In summary, density functional theory (DFT) calculations were used to study the CO oxidation on a MgO(1 0 0) supported Ptx Au3−x clusters via Langmuir–Hinshelwood (LH), trimolecular Langmuir–Hinshelwood (3LH), and Eley–Rideal (ER) mechanisms. After the comprehensive consideration of all the LH, 3LH, and ER mechanisms, the Pt2 Au1 cluster possesses the highest catalytic activity relevant to CO oxidation, which is considered to be the best catalyst among these supported subnanometer Ptx Au3−x clusters. Our results highlight a non-monotonous behavior of the catalytic activities of subnanometer clusters, and we believe that our analysis offers interesting perspectives in the understanding and exploitation of heterogeneous subnanocatalysts. Acknowledgements This work is supported by the National Natural Science Foundation of China (21176010, 91334203), Beijing Higher Education Young Elite Teacher Project, Beijing Novel Program (Z12111000250000), “Chemical Grid Project” of BUCT and Supercomputing Center of Chinese Academy of Sciences (SCCAS). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.08. 081. References [1] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide, J. Catal. 115 (1989) 301–309. [2] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0 ◦ C, Chem. Lett. (1987) 405–408. [3] P. Landon, P.J. Collier, A.J. Papworth, C.J. Kiely, G.J. Hutchings, Direct formation of hydrogen peroxide from H2 /O2 using a gold catalyst, Chem. Commun. (2002) 2058–2059. [4] M.D. Hughes, Y.-J. Xu, P. Jenkins, P. McMorn, P. Landon, D.I. Enache, A.F. Carley, G.A. Attard, G.J. Hutchings, F. King, Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions, Nature 437 (2005) 1132–1135. [5] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Active nonmetallic Au and Pt species on ceria-based water–gas shift catalysts, Science 301 (2003) 935–938. [6] Y. Gao, N. Shao, Y. Pei, Z. Chen, X.C. Zeng, Catalytic activities of subnanometer gold clusters (Au16 –Au18 , Au20 , and Au27 –Au35 ) for CO oxidation, ACS Nano 5 (2011) 7818–7829. [7] M. Haruta, Size-and support-dependency in the catalysis of gold, Catal. Today 36 (1997) 153–166. [8] M. Chen, D.W. Goodman, Catalytically active gold: from nanoparticles to ultrathin films, Acc. Chem. Res. 39 (2006) 739–746. [9] A.S.K. Hashmi, G.J. Hutchings, Gold catalysis, Angew. Chem. Int. Ed. 45 (2006) 7896–7936. [10] A. Fukuoka, P.L. Dhepe, Sustainable green catalysis by supported metal nanoparticles, Chem. Rec. 9 (2009) 224–235.
288
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288
[11] A.A. Herzing, C.J. Kiely, A.F. Carley, P. Landon, G.J. Hutchings, Identification of active gold nanoclusters on iron oxide supports for CO oxidation, Science 321 (2008) 1331–1335. [12] B. Yoon, H. Häkkinen, U. Landman, A.S. Wörz, J.-M. Antonietti, S. Abbet, K. Judai, U. Heiz, Charging effects on bonding and catalyzed oxidation of CO on Au8 clusters on MgO, Science 307 (2005) 403–407. [13] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Häkkinen, R. Barnett, U. Landman, When gold is not noble: nanoscale gold catalysts, J. Phys. Chem. A 103 (1999) 9573–9578. [14] M. Amft, N.V. Skorodumova, Catalytic activity of small MgO-supported Au clusters towards CO oxidation: a density functional study, Phys. Rev. B 81 (2010) 195443. [15] W. Zhang, D.J. Cheng, J.Q. Zhu, Theoretical study of CO catalytic oxidation on free and defective graphene-supported Au–Pd bimetallic clusters, RSC Adv. 4 (2014) 42554–42561. [16] D.J. Cheng, H.X. Xu, A. Fortunelli, Tuning the catalytic activity of Au–Pd nanoalloys in CO oxidation via composition, J. Catal. 314 (2014) 47–55. [17] F.R. Negreiros, L. Sementa, G. Barcaro, S. Vajda, E. Apra, A. Fortunelli, CO oxidation by subnanometer Agx Au3−x supported clusters via density functional theory simulations, ACS Catal. 2 (2012) 1860–1864. [18] S. Sicolo, G. Pacchioni, A DFT study of PtAu bimetallic clusters adsorbed on MgO/Ag(1 0 0) ultrathin films, Phys. Chem. Chem. Phys. 12 (2010) 6352–6356. [19] X. Bokhimi, R. Zanella, C. Angeles-Chavez, Rutile-supported Ir, Au, and Ir–Au catalysts for CO oxidation, J. Phys. Chem. C 114 (2010) 14101–14109. [20] S. Kameoka, A.P. Tsai, Oxidation behavior and catalytic property of intermetallic compound AuCu, Catal. Today 132 (2008) 88–92. [21] L.B. Ortiz-Soto, O.S. Alexeev, M.D. Amiridis, Low temperature oxidation of CO over cluster-derived platinum–gold catalysts, Langmuir 22 (2006) 3112–3117. [22] Y. Wang, J. Zhu, J. Zhang, L. Song, J. Hu, S. Ong, W. Ng, Selective oxidation of CO in hydrogen-rich mixtures and kinetics investigation on platinum–gold supported on zinc oxide catalyst, J. Power Sources 155 (2006) 440–446. [23] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation, Catal. Lett. 44 (1997) 83–87. [24] B.J. Auten, H. Lang, B.D. Chandler, Dendrimer templates for heterogeneous catalysts: bimetallic Pt–Au nanoparticles on oxide supports, Appl. Catal. B: Environ. 81 (2008) 225–235. [25] L. Sementa, G. Barcaro, F.R. Negreiros, A. Fortunelli, Ligand/cluster/support catalytic complexes in heterogeneous ultrananocatalysis: NO oxidation on Ag3 /MgO(1 0 0), Phys. Chem. Chem. Phys. 16 (2014) 26570–26577. [26] F.R. Negreiros, E. Aprà, G. Barcaro, L. Sementa, S. Vajda, A. Fortunelli, A first-principles theoretical approach to heterogeneous nanocatalysis, Nanoscale 4 (2012) 1208–1219. [27] F.R. Negreiros, G. Barcaro, L. Sementa, A. Fortunelli, Concepts in theoretical heterogeneous ultrananocatalysis, C. R. Chim. 17 (2014) 625–633. [28] F. Wang, D.J. Zhang, Y. Ding, DFT study on CO oxidation catalyzed by Ptm Aun (m + n = 4) clusters: catalytic mechanism, active component, and the configuration of ideal catalysts, J. Phys. Chem. C 114 (2010) 14076–14082. [29] D. Gerc¸eker, I. Önal, A DFT study on CO oxidation on Pd4 and Rh4 clusters and adsorbed Pd and Rh atoms on CeO2 and Ce0.75 Zr0.25 O2 supports for TWC applications, Appl. Surf. Sci. 285 (2013) 927–936.
[30] B. Kalita, R.C. Deka, Reaction intermediates of CO oxidation on gas phase Pd4 clusters: a density functional study, J. Am. Chem. Soc. 131 (2009) 13252–13254. [31] H.Y. Kim, S.S. Han, J.H. Ryu, H.M. Lee, Balance in adsorption energy of reactants steers CO oxidation mechanism of Ag13 and Ag12 Pd1 nanoparticles: association mechanism versus carbonate-mediated mechanism, J. Phys. Chem. C 114 (2010) 3156–3160. [32] D. Ma, Y. Tang, G. Yang, J. Zeng, C. He, Z. Lu, CO catalytic oxidation on iron-embedded monolayer MoS2 , Appl. Surf. Sci. 328 (2015) 71–77. [33] C. Liu, Y. Tan, S. Lin, H. Li, X. Wu, L. Li, Y. Pei, X.C. Zeng, CO self-promoting oxidation on nanosized gold clusters: triangular Au3 active site and CO induced O O scission, J. Am. Chem. Soc. 135 (2013) 2583–2595. [34] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G.L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A.P. Seitsonen, A. Smogunov, P. Umari, R.M. Wentzcovitch, QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials, J. Phys.-Condens. Mat. 21 (2009) 395502. [35] J. Perdew, Improved adsorption energetics within density functional theory using revised Perdew–Burke–Ernzerhof functionals, Phys. Rev. Lett. 77 (1996) 3865–3868. [36] D. Vanderbilt, Soft self-consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B 41 (1990) 7892. [37] R. Ismail, R. Ferrando, R.L. Johnston, Theoretical study of the structures and chemical ordering of palladium–gold nanoalloys supported on MgO(1 0 0), J. Phys. Chem. C 117 (2012) 293–301. [38] C.J. Heard, S. Vajda, R.L. Johnston, Support and oxidation effects on subnanometer palladium nanoparticles, J. Phys. Chem. C 118 (2014) 3581–3589. [39] C.J. Heard, S. Heiles, S. Vajda, R.L. Johnston, Pdn Ag (4−n) and Pdn Pt(4−n) clusters on MgO(1 0 0): a density functional surface genetic algorithm investigation, Nanoscale 16 (2014) 11777–11788. [40] G. Henkelman, B.P. Uberuaga, H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901–9904. [41] Y. Xu, A.V. Ruban, M. Mavrikakis, Adsorption and dissociation of O2 on Pt–Co and Pt–Fe alloys, J. Am. Chem. Soc. 126 (2004) 4717–4725. [42] G. Barcaro, A. Fortunelli, The interaction of coinage metal clusters with the MgO(1 0 0) surface, J. Chem. Theory Comput. 1 (2005) 972–985. [43] I. Atanasov, G. Barcaro, F.R. Negreiros, A. Fortunelli, R.L. Johnston, Modelling the metal-on-top effect for Pd clusters on the MgO(1 0 0) substrate, J. Chem. Phys. 138 (2013) 224703. [44] G. Chen, S. Li, Y. Su, V. Wang, H. Mizuseki, Y. Kawazoe, Improved stability and catalytic properties of Au16 cluster supported on graphane, J. Phys. Chem. C 115 (2011) 20168–20174. [45] Z.-P. Liu, P. Hu, A. Alavi, Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold, J. Am. Chem. Soc. 124 (2002) 14770–14779.
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CO oxidation mechanism on a MgO(1 0 0) supported Ptx Au3−x clusters Wei Zhang a,b , Rong Cui c , Hao Wu c , Jiqin Zhu b,∗∗ , Daojian Cheng a,c,∗ a
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China c Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 25 June 2015 Received in revised form 27 July 2015 Accepted 10 August 2015 Available online 12 August 2015 Keywords: CO oxidation Reaction mechanism Ptx Au3−x clusters DFT calculation
a b s t r a c t In this work, we systematically studied the CO oxidation on a MgO(1 0 0) supported Ptx Au3−x clusters via Langmuir–Hinshelwood (LH), trimolecular Langmuir–Hinshelwood (3LH), and Eley–Rideal (ER) mechanisms on the basis of density functional theory (DFT) calculations. It is found that the Pt2 Au1 /MgO cluster possesses the highest catalytic activity relevant to CO oxidation, which is considered to be the best catalyst among these supported subnanometer Ptx Au3−x clusters. We expect that computational screening of mechanism-dependent properties of sub-nanocatalysts in this work could be useful in the design of highly efficient nanocatalysts. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nanosized gold clusters have attracted great interest over the past few decades owing to their unusual catalytic properties not seen in bulk gold. For example, nanosized gold clusters exhibit high catalytic activities toward the carbon monoxide oxidation [1,2], selective oxidation of olefin and alcohol [3], synthesis of hydrogen peroxide [4], and water–gas shift reaction [5]. Among various reactions catalyzed by gold clusters, CO oxidation has attracted great attention and become a prototypical reaction for examining the activities of nanosized gold clusters [6–10]. More interestingly, the activity of gold clusters relevant to CO oxidation is strongly dependent on the cluster size. After Haruta’s discovery that supported gold nanoclusters are active in CO oxidation at low temperature [2], a lot of studies have flourished on this topic. More interestingly, the active catalyst for CO oxidation can be subnanometer species containing around 10 Au atoms [11]. Landman et al. found that the supported Au8 cluster is highly active for CO oxidation by both experimental and theoretical studies [12,13]. In addition, Skorodumova et al. studied the catalytic activity of MgO(1 0 0)-supported Au1–4 clusters toward CO oxidation by density functional theory
∗ Corresponding author at: State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Zhu), [email protected] (D. Cheng). http://dx.doi.org/10.1016/j.apsusc.2015.08.081 0169-4332/© 2015 Elsevier B.V. All rights reserved.
(DFT) calculations [14]. These findings can trigger our studies on CO oxidation on supported small (subnanometer) Au clusters. In recent years, bimetallic clusters have attracted a lot of attention, since the introduction of a second metal into the single metal cluster can improve the catalytic activity of the clusters by the synergetic effect of two metals [15–17]. Notably, Au-based bimetallic clusters by the mixing of Au with other metals, such as Pt [15,18], Pd [16], Ag [17], Ir [19], and Cu [20], have been used to improve the catalytic efficiency, stability, and selectivity of pure Au clusters for CO oxidation. Among these systems, Pt–Au bimetallic clusters have been found experimentally to show excellent catalytic performance for CO oxidation [21–24]. However, only a few studies have been focused on the catalytic properties of bimetallic subnanometer clusters for CO oxidation [16,17,25–27]. The pioneering work of Fortunelli et al. studied the catalytic properties of Agx Au3−x clusters supported on the MgO(1 0 0) surface [17], and demonstrated that Ag2 Au1 cluster is a good catalyst for CO oxidation in terms of efficiency and stability. In addition, Ding et al. studied the catalytic properties of CO oxidation on free Ptm Aun (m + n = 4) clusters by DFT calculations [28], and found that free Pt–Au clusters are more active than the corresponding pure Pt4 cluster for CO oxidation. To the best of our knowledge, the theoretical understanding of the reaction mechanism of CO oxidation on supported subnanometer Pt–Au clusters is still lacking and the details of how the support and cluster composition affect the reaction mechanisms is also an open question. DFT calculation is an established tool to evaluate the catalytic properties of the clusters via different reaction mechanisms for CO oxidation [29]. In general, two well-established reaction
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288
mechanisms, namely Langmuir–Hinshelwood (LH) and Eley–Rideal (ER), are considered for CO oxidation [17,30–32]. For the LH mechanism, the coadsorbed CO and O2 firstly form a peroxo-type complex intermediate, i.e., CO + O2 → OOC → O + CO2 , then the intermediate OOCO is critical for CO oxidation, while for the ER mechanism, CO reacts directly with activated O2 , but the precondition is that O2 is easily dissociated to O atom, i.e., CO + O → CO2 . Moreover, Zeng et al. found that the coadsorbed CO molecule at a unique triangular Au3 active site can act as a promoter for the scission of an O O bond, leading to the spontaneous formation (due to the extremely low energy barrier) of two CO2 molecules as product, i.e., 2CO + O2 → OCOOCO → 2CO2 , which is named as the trimolecular Langmuir–Hinshelwood (3LH) mechanism [33]. On the theoretical side, all the three mechanisms could take place on the catalyst. However, detailed accurate studies involving all the three mechanisms for CO oxidation reaction on a selected system have been largely overlooked. Thus, there is a need to explore all the possible intermediate products and reaction pathways for CO oxidation reaction on heterogeneous subnanometer catalysts, such as supported subnanometer Pt–Au clusters. In this work, a complete and systematic study of CO oxidation on the Ptx Au3−x trimers supported on the MgO(1 0 0) surface via LH, 3LH, and ER mechanisms are studied by DFT calculations. In particular, the adsorption energies of reactants (O2 , CO, 2CO, CO + O2 , and 2CO + O2 ), intermediates (OOCO, CO + OOCO, and COOOCO), and energy barriers of three mechanisms on the MgO(1 0 0)-supported Ptx Au3−x trimmers are calculated. The effect of support and cluster composition on the reaction mechanisms is discussed.
2. Computational details DFT calculations were performed using the PWSCF (PlaneWave Self-Consistent Field) plane wave code in the Quantum ESPRESSO package [34]. All the calculations were carried out by the spin-polarized generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) [35] XC-functionals together with ultrasoft pseudopotentials (see Supporting Information for details) [36]. The ultrasoft pseudopotentials used in this work were generated with a Scalar-relativistic calculation, which include the nonlinear core correction (NLCC) and relativistic effect. The MgO(1 0 0) surface was modeled in a supercell approach with 2ML thick 3 × 3 MgO slab. In order to test the convergence of the adsorption energies of Ptx Au3−x clusters on MgO slab with different layers, the adsorption energies of Ptx Au3−x cluster on MgO with different layers were calculated, as listed in Table S1. It is found that the energies changes are less than 0.03 eV using two or three layers for MgO slab, which is within the DFT error bar. Therefore, the 2ML thickness for MgO slab is acceptable and used in this work. The repeated slabs were separated by 20 A˚ vacuum region to avoid the interaction between two successive slabs. All the MgO atoms in the unit cell were kept frozen during structural optimizations, since the flat MgO substrate is quite rigid [17,37–39]. Spin-polarized calculations were performed here by using values of 40 and 400 Ry as the energy cut-off for the selection of the plane waves for the description of the wave function and the electronic density, respectively, which are considered to be reasonable after the convergence test (see Fig. S1). The Brillouin zone sampled at the Gamma-point only, and the geometry of the cluster upon or without adsorption is optimized until the total energy is converged to 10−6 eV. We calculated the adsorption energies of CO and O2 according to the equation: Eads = Etotal − Ecluster/MgO − Eadsorbate
(1)
283
Furthermore, the coadsorption energies of CO and the molecular or atomic oxygen were calculated as: Ecoads = Etotal − Ecluster/MgO − Eco − Eo2 (or O)
(2)
In the above equations, Etotal , Ecluster/MgO , Eabsorbate , Eco , and EO2 (or O) correspond to the electronic energies of adsorbed species on the supported cluster, the bare supported cluster, a gas-phase adsorbate, gaseous CO and gaseous O2 (or O), respectively. The climbing-image nudged elastic band (CI-NEB) method [40] was adopted here to determine the reaction pathway for the CO oxidation. In the CI-NEB calculations, the reaction paths are interpolated by fitting a cubic polynomial through all images on each path guided by the force tangent to the reaction coordinate at each image. Once a minimum-energy path is determined, the transition state is located [41]. The energies along the paths were referred to the sum of energies of the noninteracting cluster and reactant(s), and the barrier heights along them representing the activation energies were calculated as Eb = E TS − E IS , ETS
(3) EIS
where and are the total energies of the transition and initial states, respectively. 3. Results and discussion 3.1. The structure of the cluster In this work, we optimized the triangular clusters of the supported Ptx Au3−x clusters with two configurations: (a) the planar configuration, (b) the “metal-on-top” configuration, as shown in Fig. S2. Our results certify that the planar configuration on the MgO surface is unstable, which would be changed into the “metal-ontop” one after optimization. This finding is in excellent agreement with many previous works [18,25,42,43]. In order to find the stable structure for these supported clusters, all the “metal-on-top” configurations of the Ptx Au3−x trimmers on the MgO surface were studied. The available adsorption configurations and the corresponding adsorption energies of the Ptx Au3−x trimers on the MgO surface are shown in Fig. S3. Only one “metal-on-top” configuration is found for the supported Au3 or Pt3 clusters, while two different “metal-on-top” configurations are found for the supported Au2 Pt1 and Au1 Pt2 clusters. In addition, all the most stable adsorption configurations and the corresponding adsorption energies of the Ptx Au3−x trimmers on the MgO surface are shown in Fig. 1. The adsorption energies for the Pt3 , Pt2 Au1 , Au2 Pt1 , and Au3 clusters on the MgO(1 0 0) surface are −2.73, −2.98, −2.18, and −1.42 eV, respectively, indicating that the adsorption strength of these clusters on the MgO(1 0 0) surface follows the order: Pt2 Au1 > Pt3 > Au2 Pt1 > Au3 . It means that alloying with Pt can increase the adsorption strength of the Au3 cluster on the MgO(1 0 0) surface. Another crucial feature of the catalyst is its stability under reaction conditions. To check this aspect of the supported Ptx Au3−x trimers, the energy barriers for breaking Ptx Au3−x CO and Ptx Au3−x O2 complexes on the MgO surfaces were calculated, as listed in Table 1. The corresponding initial and final configurations are shown in Fig. S4. The energy barriers for breaking the bare Pt3 , Pt2 Au1 , Au2 Pt1 , and Au3 clusters were also calculated for comparison, as listed in Table 1. It is found that the difficulty for breaking the supported Ptx Au3−x CO and Ptx Au3−x O2 complexes follows the order: Pt3 > Pt2 Au1 > Au2 Pt1 > Au3 . Upon the adsorption of CO/O2 , the supported Pt3 and Pt2 Au1 clusters are found to be still reasonably stable, whereas the supported Au3 and Pt1 Au2 clusters could break easily. Our results show that Pt-rich trimers should have a longer lifetime under reaction conditions of CO oxidation.
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Fig. 1. The most favorable adsorption configurations of (a) Au3 /MgO, (b) Pt1 Au2 /MgO, (c) Pt2 Au1 /MgO, and (d) Pt3 /MgO clusters. Ead denotes their adsorption energy on Mg(1 0 0) and M is their preferred spin multiplicity; Au, Pt, Mg, and O atoms are represented by yellow, blue, green, and red spheres, respectively (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Table 1 The energy barriers (Eb in eV) for breaking the Ptx Au3−x /MgO clusters into smaller pieces with or without the adsorption of CO or O2. Eb
Ptx Au3−x
Ptx Au3−x CO
Ptx Au3−x O2
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
0.79 1.96 2.45 2.88
0.53 0 2.24 9.53
1.14 1.43 1.96 2.46
3.2. The adsorption of CO and O2 As illustrated in Fig. 2a, the surface of a supported Ptx Au3−x trimer is a triangle. On the triangle, seven different adsorption sites are possible: three T (on the top of three vertex atoms) sites, three B (on the bridge between two vertex atoms) sites, and one H (on the hollow among three vertex atoms) site. We start by conducting a preliminary investigation on CO adsorption on the supported Ptx Au3−x trimers at each different composition. For CO adsorption, the molecule prefers the “end-on” configurations with C atom bind to the Pt or Au atoms, as shown in Fig. 2b. Except the Au2 Pt1 /MgO cluster, which has three different T sites, the other supported Ptx Au3−x clusters have only two different T sites. All the adsorption sites and the corresponding adsorption energies of CO and 2CO on the supported Ptx Au3−x trimmers are listed in Table S2. All the most stable adsorption configurations and the corresponding adsorption energies of CO on the supported Ptx Au3−x trimmers are listed in Table 2. It is found that the T1 site is the most stable one for CO adsorption on the Au3 /MgO cluster with the adsorption energy of −1.39 eV. Accordingly, the T3 site is the most favorable one for CO adsorption on the Au2 Pt1 /MgO cluster with the adsorption energy of −1.96 eV. In addition, the T2 site is the most stable one for CO adsorption on the Pt2 Au1 /MgO and Pt3 /MgO clusters with the adsorption energies of −2.25 and −3.57 eV, respectively. For O2 adsorption on these supported clusters, the O2 molecule prefers the “side-on” configurations on B sites and the “end-on” configurations on the T and H sites, as shown in Fig. 2b. The available adsorption configurations and the corresponding adsorption energies of O2 on the supported Ptx Au3−x trimmers are summarized in Table S3. It is found that the B1 configuration is the most stable one for O2 adsorption on the Au3 /MgO and Au2 Pt1 /MgO clusters with the adsorption energies of −1.03 and −1.05 eV, respectively. In contrast, the T2 configuration is the most stable one for O2 adsorption on the Pt2 Au1 /MgO cluster with the adsorption energy of −1.19 eV. For O2 adsorption on the Pt3 /MgO cluster, the B2 configuration is the most stable one with the adsorption energy of −2.16 eV. All the most stable adsorption configurations and the corresponding adsorption energies of O2 on the supported Ptx Au3−x trimmers are listed in Table 2. In order to identify the spin states of the O2 adsorption intermediates involved in the reaction pathway, the spin states of O2 in the ground state and the reaction
pathway for the Pt2 Au1 /MgO cluster are listed in Table S4. It is found that the interaction of O2 with the metal atoms can shift the triplet ground state 3 O2 into the doublet 2 O2 − (or singlet 1 O2 − ), which is well-known to be highly reactive [44]. 3.3. CO oxidation For CO oxidation, a bimolecular Langmuir–Hinshelwood (LH) mechanism is investigated firstly. The potential energy profile and configurations for CO oxidation on Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters with the LH mechanism are shown in Fig. 3a–d, respectively. For clarity, the corresponding adsorption energies and energy barriers on these clusters with the LH mechanism are listed in Table 3. As mentioned above, both CO and O2 molecules could adsorb on the Ptx Au3−x /MgO clusters, which would provide the initial stage for the catalytic reaction of CO oxidation. After the check, the favorable coadsorption site for CO and O2 molecules on the Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters is found to be T1 for O2 and T3 for CO, as listed in Table 2. Moreover, the favorable coadsorption site for CO and O2 molecules on the Au3 /MgO cluster are found to be T1 for O2 and T1 for CO (see Table 2). In the following, CO and O2 move closer to form the intermediate state OCOO* with a C O single bond and a peroxide O O bond, followed by crossing of the first energy barrier (TS1). The corresponding energy barriers (TS1) are found to be 0.17, 0.63, 0.47, and 0.50 eV for the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters, respectively. It is noted that the energy barrier for the formation of the intermediate state OCOO* (TS1) on the Au3 /MgO cluster is lowest among these clusters. In the third step, the peroxide O O bond dissociates into two O atoms by crossing the second energy barrier (TS2). For the Au3 /MgO cluster, the energy barrier (TS2) is 0.29 eV, which is bigger than its energy barrier for the formation of OCOO* (TS1). For the Au2 Pt1 /MgO cluster, the value of energy barrier (TS2) is 0.11 eV, which is much lower than that of its first energy barrier (TS1). In contrast, the calculated energy barriers (TS2) are zero for the Pt2 Au1 /MgO and Pt3 /MgO clusters. It means that CO2 molecule is easily desorbed from the supported Pt-rich clusters without energy barrier. For CO oxidation on these supported clusters with the LH mechanism, the Au3 /MgO cluster has the lowest energy barrier for CO oxidation on these supported subnanometer Ptx Au3−x clusters, and the activity of CO oxidation follows the order: Au3 /MgO > Pt2 Au1 /MgO > Pt3 /MgO > Pt1 Au2 /MgO. The trimolecular Langmuir–Hinshelwood (3LH) mechanism was also investigated, which is also called CO self-promoting oxidation mechanism [33]. In this mechanism, the coadsorbed CO molecule at a unique triangular Au3 active site can act as a promoter for the scission of an O O bond, leading to the spontaneous formation of two CO2 molecules. In order to investigate the 3LH reaction mechanism, the adsorption of two CO molecules on these supported clusters was also calculated, as listed in Table 2. For the
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288
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Fig. 2. (a) Atomic adsorption sites for the triangular facet of Pt1 Au2 cluster. For the nomenclature of adsorption sites of T1, T2, T3, B1, B2, B3, H, please see the text. (b) Adsorption mode of CO on the T site with “end-on” configuration, adsorption mode of O2 on the B1 site with “side-on” configuration and the H site with “end-on” configuration. Table 2 The most stable adsorption sites and the corresponding adsorption energies (Ead in eV) of CO and O2 , and coadsorption of 2CO, CO + O2 , and 2CO + O2 on these Ptx Au3−x /MgO clusters. Sites labeled “N.A.” are unstable.
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
Adsorption site Ead Adsorption site Ead Adsorption site Ead Adsorption site Ead
O2
CO
2CO
CO + O2
2CO + O2
B1 −1.03 B1 −0.88 T2 −1.19 B1 −2.16
T1 −1.39 T3 −1.96 T2 −2.25 T2 −3.57
T2 + T3 −2.09 T1 + T3 −3.58 T2 + T3 −4.19 T1 + T2 −6.18
T1 + T1 −1.60 T1 + T3 −2.65 T1 + T3 −2.57 T1 + T3 −4.13
N.A. – N.A. – T1 + T2 + T3 −6.45 B1 + T1 + T3 −6.42
Fig. 3. Potential energy profile and configurations for CO oxidation with the Langmuir–Hinshelwood (LH) mechanism on the (a) Au3 /MgO, (b) Pt1 Au2 /MgO, (c) Pt2 Au1 /MgO, and (d) Pt3 /MgO clusters. Table 3 Calculated relative adsorption energies (in eV) of initial adsorption (CO), coadsorption (CO + O2 ), intermediate (OCOO), the final (CO2 + O) states, and the energy barriers (in eV) of the first (TS1) and second (TS2) transition states of CO oxidation reaction with the Langmuir–Hinshelwood (LH) mechanism on these Ptx Au3−x /MgO clusters.
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
CO
CO + O2
OOCO
O + CO2
TS1(->OCOO)
TS2(->CO2 + O)
−1.39 −2.40 −2.25 −3.57
−1.60 −2.65 −2.57 −4.13
−1.59 −2.11 −1.79 −3.54
−3.55 −2.98 −3.21 −5.06
0.17 0.63 0.47 0.50
0.29 0.11 0 0
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Fig. 4. Potential energy profile and configurations for CO oxidation with the trimolecular Langmuir–Hinshelwood (3LH) mechanism on the (a) Pt1 Au2 /MgO and (b) Au3 /MgO clusters. Table 4 Calculated relative adsorption energies (in eV) of initial adsorption (CO), the coadsorption (2CO/2CO + O2 ), intermediate (CO + OCOO/COOOCO), the final (2CO2 ) states, and the energy barriers (in eV) of the first (TS1) and second (TS2) transition states of CO oxidation reaction with the trimolecular Langmuir–Hinshelwood (3LH) mechanism on the Pt2 Au1 /MgO and Pt3 /MgO clusters.
Pt2 Au1 /MgO Pt3 /MgO
CO
2CO
2CO + O2
CO + OOCO
COOOCO
2CO2
TS1(->OCOO)
TS2(->COOOCO)
−2.25 −3.57
−4.19 −6.18
−6.45 −6.42
−5.74 −5.74
−5.14 −5.31
−6.03 −8.66
0.69 0.71
0.67 1.29
Fig. 5. Potential energy profile and configurations for CO oxidation with the Eley–Rideal (ER) mechanism on the (a) Au3 /MgO, (b) Pt1 Au2 /MgO, (c) Pt2 Au1 /MgO, and (d) Pt3 /MgO clusters.
Au3 /MgO and Pt2 Au1 /MgO clusters, the T2 and T3 sites are the stable adsorption ones for the coadsorption of two CO molecules, while the most stable adsorption sites are T1 and T3 ones for the Au2 Pt1 /MgO and Pt3 /MgO clusters. In addition, we also calculated the coadsorption of two CO molecules and one O2 molecule. It is found that the coadsorption of two CO molecules can promote the
adsorption of O2 for the Pt2 Au1 /MgO cluster, but the coadsorption of two CO molecules can inhibit the adsorption of O2 molecule for the Pt3 /MgO cluster, as listed in Table 2. It means that the coadsorption of two CO molecules on the surface of small clusters can significantly change the structure and the reactivity of the cluster toward the O2 molecule. Moreover, the coadsorption of two CO
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288 Table 5 Calculated relative adsorption energies (in eV) of initial adsorption (O2 ), the coadsorption (2O), the final (2CO2 ) states, and the energy barriers (in eV) of the first (TS1) transition states of CO oxidation reaction with the Eley–Rideal (ER) mechanism on the Ptx Au3−x /MgO clusters.
Au3 /MgO Au2 Pt1 /MgO Pt2 Au1 /MgO Pt3 /MgO
O2
O O
2CO2
TS1(->2O)
−1.03 −0.88 −0.38 −2.16
0.16 −0.11 −3.11 −3.11
−5.99 −6.65 −6.03 −8.66
2.12 0.96 0.22 0.34
molecules and one O2 molecule on the Au3 /MgO and Au2 Pt1 /MgO clusters is impossible, meaning that the 3LH reaction mechanism is not available for the Au3 /MgO and Au2 Pt1 /MgO clusters. In contrast, the 3LH mechanism can occur on Au clusters with tens of atoms [33], indicating that the size of the cluster plays an important role in this mechanism. The potential energy profile and configurations for CO oxidation on the Pt2 Au1 /MgO and Pt3 /MgO clusters with the 3LH mechanism are shown in Fig. 4a and b, respectively. For clarity, the corresponding adsorption energies and energy barriers on these supported clusters with the 3LH mechanism are listed in Table 4. It is found that the coadsorbed CO molecule cannot promote the O O bond breaking via 3LH mechanism for the Pt2 Au1 /MgO and Pt3 /MgO clusters, since the activation energy of O O bond dissociation via 3LH mechanisms is 0 eV. The energy barriers for the 3LH mechanism are bigger than that of the previous LH mechanism. Our results show that the 3LH mechanism is not a universally applicable mechanism for these supported subnanometer Ptx Au3−x clusters. We also investigated the Eley–Rideal (ER) mechanism on these supported subnanometer Ptx Au3−x clusters. For the ER mechanism, the following step after O2 breaking is the reaction of an O atom with CO, in which the dissociation of O2 on the clusters is the critical process. The potential energy profile and configurations for CO oxidation on the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters with the ER mechanism are shown in Fig. 5a–d, respectively. The corresponding adsorption energies and energy barriers on these supported clusters with the ER mechanism are also given in Table 5. It is found that the dissociation energy barriers of O2 (TS1) are 2.12, 0.96, 0.22, and 0.34 eV for the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters, respectively. It means that the dissociation of O2 is really difficult on the supported Au-rich clusters, which is in excellent agreement with the previous work [45]. In the following, the O atom reacts with the gas CO molecule by crossing the second energy barrier (TS2). The calculated TS2 energy barriers are zero for these supported subnanometer Ptx Au3−x clusters. It means that CO2 molecule is easily formed without energy barrier in the ER mechanism. For CO oxidation on these supported clusters with the ER mechanism, the Pt2 Au1 /MgO cluster has the lowest energy barrier for CO oxidation on these supported subnanometer Ptx Au3−x clusters, and the activity of CO oxidation follows the order: Pt2 Au1 /MgO > Pt3 /MgO > Au2 Pt1 /MgO > Au3 /MgO. To conclude this section, we briefly discuss the three different reaction mechanisms, namely LH, 3LH, and ER, for CO oxidation on the Au3 /MgO, Au2 Pt1 /MgO, Pt2 Au1 /MgO, and Pt3 /MgO clusters. It is found that 3LH mechanism cannot promote the CO oxidation reactivity on the supported subnanometer Pt2 Au1 and Pt3 clusters. For the LH mechanism, the Au3 /MgO cluster has the lowest energy barrier of 0.29 eV for CO oxidation on these supported subnanometer Ptx Au3−x clusters. For the ER mechanism, the Pt2 Au1 /MgO cluster has the lowest energy barrier of 0.22 eV for CO oxidation on these supported subnanometer Ptx Au3−x clusters. Considering the LH, 3LH, and ER mechanisms, the Pt2 Au1 cluster possesses the highest catalytic activity relevant to CO oxidation, which is considered to be the best catalyst among these supported subnanometer Ptx Au3−x clusters.
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It is well-known that the activity of the catalyst to a specific reaction is largely dictated by the local electronic environments of the catalyst. In this work, the Pt2 Au1 /MgO cluster possesses the higher catalytic activity relevant to CO oxidation, compared with the Au3 /MgO cluster. To understand the reason, the Löwdin electronic population was used to analyze the local electronic environments of the Ptx Au3−x /MgO clusters. Fig. S5 shows the number of the total charge of Pt and Au atoms for these supported Ptx Au3−x clusters. It is found that more charge transfer from Au to Pt atoms takes place in Pt–Au bimetallic clusters, compared with the pure Au3 and Pt3 clusters. It is well-known that the charge transfer between Au and Pt atoms can improve the activity of the Pt–Au bimetallic clusters, which may be the reason that the supported Pt2 Au1 cluster is more active than that of the supported Pt3 cluster. 4. Conclusion In summary, density functional theory (DFT) calculations were used to study the CO oxidation on a MgO(1 0 0) supported Ptx Au3−x clusters via Langmuir–Hinshelwood (LH), trimolecular Langmuir–Hinshelwood (3LH), and Eley–Rideal (ER) mechanisms. After the comprehensive consideration of all the LH, 3LH, and ER mechanisms, the Pt2 Au1 cluster possesses the highest catalytic activity relevant to CO oxidation, which is considered to be the best catalyst among these supported subnanometer Ptx Au3−x clusters. Our results highlight a non-monotonous behavior of the catalytic activities of subnanometer clusters, and we believe that our analysis offers interesting perspectives in the understanding and exploitation of heterogeneous subnanocatalysts. Acknowledgements This work is supported by the National Natural Science Foundation of China (21176010, 91334203), Beijing Higher Education Young Elite Teacher Project, Beijing Novel Program (Z12111000250000), “Chemical Grid Project” of BUCT and Supercomputing Center of Chinese Academy of Sciences (SCCAS). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2015.08. 081. References [1] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide, J. Catal. 115 (1989) 301–309. [2] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0 ◦ C, Chem. Lett. (1987) 405–408. [3] P. Landon, P.J. Collier, A.J. Papworth, C.J. Kiely, G.J. Hutchings, Direct formation of hydrogen peroxide from H2 /O2 using a gold catalyst, Chem. Commun. (2002) 2058–2059. [4] M.D. Hughes, Y.-J. Xu, P. Jenkins, P. McMorn, P. Landon, D.I. Enache, A.F. Carley, G.A. Attard, G.J. Hutchings, F. King, Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions, Nature 437 (2005) 1132–1135. [5] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Active nonmetallic Au and Pt species on ceria-based water–gas shift catalysts, Science 301 (2003) 935–938. [6] Y. Gao, N. Shao, Y. Pei, Z. Chen, X.C. Zeng, Catalytic activities of subnanometer gold clusters (Au16 –Au18 , Au20 , and Au27 –Au35 ) for CO oxidation, ACS Nano 5 (2011) 7818–7829. [7] M. Haruta, Size-and support-dependency in the catalysis of gold, Catal. Today 36 (1997) 153–166. [8] M. Chen, D.W. Goodman, Catalytically active gold: from nanoparticles to ultrathin films, Acc. Chem. Res. 39 (2006) 739–746. [9] A.S.K. Hashmi, G.J. Hutchings, Gold catalysis, Angew. Chem. Int. Ed. 45 (2006) 7896–7936. [10] A. Fukuoka, P.L. Dhepe, Sustainable green catalysis by supported metal nanoparticles, Chem. Rec. 9 (2009) 224–235.
288
W. Zhang et al. / Applied Surface Science 356 (2015) 282–288
[11] A.A. Herzing, C.J. Kiely, A.F. Carley, P. Landon, G.J. Hutchings, Identification of active gold nanoclusters on iron oxide supports for CO oxidation, Science 321 (2008) 1331–1335. [12] B. Yoon, H. Häkkinen, U. Landman, A.S. Wörz, J.-M. Antonietti, S. Abbet, K. Judai, U. Heiz, Charging effects on bonding and catalyzed oxidation of CO on Au8 clusters on MgO, Science 307 (2005) 403–407. [13] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Häkkinen, R. Barnett, U. Landman, When gold is not noble: nanoscale gold catalysts, J. Phys. Chem. A 103 (1999) 9573–9578. [14] M. Amft, N.V. Skorodumova, Catalytic activity of small MgO-supported Au clusters towards CO oxidation: a density functional study, Phys. Rev. B 81 (2010) 195443. [15] W. Zhang, D.J. Cheng, J.Q. Zhu, Theoretical study of CO catalytic oxidation on free and defective graphene-supported Au–Pd bimetallic clusters, RSC Adv. 4 (2014) 42554–42561. [16] D.J. Cheng, H.X. Xu, A. Fortunelli, Tuning the catalytic activity of Au–Pd nanoalloys in CO oxidation via composition, J. Catal. 314 (2014) 47–55. [17] F.R. Negreiros, L. Sementa, G. Barcaro, S. Vajda, E. Apra, A. Fortunelli, CO oxidation by subnanometer Agx Au3−x supported clusters via density functional theory simulations, ACS Catal. 2 (2012) 1860–1864. [18] S. Sicolo, G. Pacchioni, A DFT study of PtAu bimetallic clusters adsorbed on MgO/Ag(1 0 0) ultrathin films, Phys. Chem. Chem. Phys. 12 (2010) 6352–6356. [19] X. Bokhimi, R. Zanella, C. Angeles-Chavez, Rutile-supported Ir, Au, and Ir–Au catalysts for CO oxidation, J. Phys. Chem. C 114 (2010) 14101–14109. [20] S. Kameoka, A.P. Tsai, Oxidation behavior and catalytic property of intermetallic compound AuCu, Catal. Today 132 (2008) 88–92. [21] L.B. Ortiz-Soto, O.S. Alexeev, M.D. Amiridis, Low temperature oxidation of CO over cluster-derived platinum–gold catalysts, Langmuir 22 (2006) 3112–3117. [22] Y. Wang, J. Zhu, J. Zhang, L. Song, J. Hu, S. Ong, W. Ng, Selective oxidation of CO in hydrogen-rich mixtures and kinetics investigation on platinum–gold supported on zinc oxide catalyst, J. Power Sources 155 (2006) 440–446. [23] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation, Catal. Lett. 44 (1997) 83–87. [24] B.J. Auten, H. Lang, B.D. Chandler, Dendrimer templates for heterogeneous catalysts: bimetallic Pt–Au nanoparticles on oxide supports, Appl. Catal. B: Environ. 81 (2008) 225–235. [25] L. Sementa, G. Barcaro, F.R. Negreiros, A. Fortunelli, Ligand/cluster/support catalytic complexes in heterogeneous ultrananocatalysis: NO oxidation on Ag3 /MgO(1 0 0), Phys. Chem. Chem. Phys. 16 (2014) 26570–26577. [26] F.R. Negreiros, E. Aprà, G. Barcaro, L. Sementa, S. Vajda, A. Fortunelli, A first-principles theoretical approach to heterogeneous nanocatalysis, Nanoscale 4 (2012) 1208–1219. [27] F.R. Negreiros, G. Barcaro, L. Sementa, A. Fortunelli, Concepts in theoretical heterogeneous ultrananocatalysis, C. R. Chim. 17 (2014) 625–633. [28] F. Wang, D.J. Zhang, Y. Ding, DFT study on CO oxidation catalyzed by Ptm Aun (m + n = 4) clusters: catalytic mechanism, active component, and the configuration of ideal catalysts, J. Phys. Chem. C 114 (2010) 14076–14082. [29] D. Gerc¸eker, I. Önal, A DFT study on CO oxidation on Pd4 and Rh4 clusters and adsorbed Pd and Rh atoms on CeO2 and Ce0.75 Zr0.25 O2 supports for TWC applications, Appl. Surf. Sci. 285 (2013) 927–936.
[30] B. Kalita, R.C. Deka, Reaction intermediates of CO oxidation on gas phase Pd4 clusters: a density functional study, J. Am. Chem. Soc. 131 (2009) 13252–13254. [31] H.Y. Kim, S.S. Han, J.H. Ryu, H.M. Lee, Balance in adsorption energy of reactants steers CO oxidation mechanism of Ag13 and Ag12 Pd1 nanoparticles: association mechanism versus carbonate-mediated mechanism, J. Phys. Chem. C 114 (2010) 3156–3160. [32] D. Ma, Y. Tang, G. Yang, J. Zeng, C. He, Z. Lu, CO catalytic oxidation on iron-embedded monolayer MoS2 , Appl. Surf. Sci. 328 (2015) 71–77. [33] C. Liu, Y. Tan, S. Lin, H. Li, X. Wu, L. Li, Y. Pei, X.C. Zeng, CO self-promoting oxidation on nanosized gold clusters: triangular Au3 active site and CO induced O O scission, J. Am. Chem. Soc. 135 (2013) 2583–2595. [34] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G.L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A.P. Seitsonen, A. Smogunov, P. Umari, R.M. Wentzcovitch, QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials, J. Phys.-Condens. Mat. 21 (2009) 395502. [35] J. Perdew, Improved adsorption energetics within density functional theory using revised Perdew–Burke–Ernzerhof functionals, Phys. Rev. Lett. 77 (1996) 3865–3868. [36] D. Vanderbilt, Soft self-consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B 41 (1990) 7892. [37] R. Ismail, R. Ferrando, R.L. Johnston, Theoretical study of the structures and chemical ordering of palladium–gold nanoalloys supported on MgO(1 0 0), J. Phys. Chem. C 117 (2012) 293–301. [38] C.J. Heard, S. Vajda, R.L. Johnston, Support and oxidation effects on subnanometer palladium nanoparticles, J. Phys. Chem. C 118 (2014) 3581–3589. [39] C.J. Heard, S. Heiles, S. Vajda, R.L. Johnston, Pdn Ag (4−n) and Pdn Pt(4−n) clusters on MgO(1 0 0): a density functional surface genetic algorithm investigation, Nanoscale 16 (2014) 11777–11788. [40] G. Henkelman, B.P. Uberuaga, H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901–9904. [41] Y. Xu, A.V. Ruban, M. Mavrikakis, Adsorption and dissociation of O2 on Pt–Co and Pt–Fe alloys, J. Am. Chem. Soc. 126 (2004) 4717–4725. [42] G. Barcaro, A. Fortunelli, The interaction of coinage metal clusters with the MgO(1 0 0) surface, J. Chem. Theory Comput. 1 (2005) 972–985. [43] I. Atanasov, G. Barcaro, F.R. Negreiros, A. Fortunelli, R.L. Johnston, Modelling the metal-on-top effect for Pd clusters on the MgO(1 0 0) substrate, J. Chem. Phys. 138 (2013) 224703. [44] G. Chen, S. Li, Y. Su, V. Wang, H. Mizuseki, Y. Kawazoe, Improved stability and catalytic properties of Au16 cluster supported on graphane, J. Phys. Chem. C 115 (2011) 20168–20174. [45] Z.-P. Liu, P. Hu, A. Alavi, Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold, J. Am. Chem. Soc. 124 (2002) 14770–14779.